Pb—Sn alloy coated steel sheets can exhibit excellent corrosion resistance, formability, solderability (weldability), and the like, and have been used as materials for fuel tanks, for example, for vehicle fuel tanks. Although Pb—Sn alloy coated steel sheets can be used as materials for vehicle fuel tanks due to their excellent properties (for example, seam weldability and the like), Pb-free materials may be preferable for reasons relating, e.g., to the global environment.
Sn—Zn alloy coated steel sheets can be manufactured using electroplating techniques for electrolyzing a bare steel sheet in an aqueous solution containing Zn and Sn ions as described, for example, in Japanese Patent Publication No. S52-130438. Sn—Zn alloy coated steel sheets having Sn as a main component can be used for electronic parts due to their excellent corrosion resistance and solderability. Such Sn—Zn alloy coated steel sheets can exhibit excellent properties for use in vehicle fuel tanks.
For example, hot-dip Sn—Zn system coated steel sheets are described, e.g., in Japanese Patent Nos. 3126622 and 3126623, and in International Publication No. WO 96/30560. Such hot-dip Sn—Zn system coated steel sheets can exhibit reliable corrosion resistance, formability and solderability. However, there may be a need to further improve corrosion resistance. For example, pitting corrosion due to Zn segregation can occur even in an unprocessed flat portion of a Sn—Zn coated steel sheet. For example, because red rust can be produced in a short time in a salt spray test which can be similar to salt-damaging environments, there may be an insufficient corrosion resistance in salt-damaging environments. The amount of Zn in such materials may be increase to further improve a sacrificial corrosion resistance effect. However, if such addition of Zn is excessive, a main component of a coating layer may transition from Sn to Zn. Thus, Zn can be eluted even more than Sn, which may result in a deterioration of corrosion resistance of the coating layer.